Chemistry Reference
In-Depth Information
In Chapter 7, we learned how to do numerical calculations for compounds, using
their formulas as a basis. This chapter lays the foundation for doing similar cal-
culations for chemical reactions, using the balanced equation as a basis. The
chemical equation is introduced in Section 8.1, and methods for balancing equa-
tions are presented in Section 8.2. To write equations, we must often be able to
predict the products of a reaction from a knowledge of the properties of the
reactants. Section 8.3 shows how to classify chemical reactions into types to
predict the products of thousands of reactions. An important type of reaction—
the acid-base reaction—is discussed in Section 8.4.
8.1
The Chemical Equation
In a chemical reaction, the substances that react are called
reactants,
or some-
times,
reagents.
The substances that are produced are called
products.
During
the reaction, some or all of the atoms of the reactants change their bonding.
For example, two hydrogen molecules can react with an oxygen molecule to
form two water molecules (Figure 8.1). In a less familiar example—the reac-
tion of with —the products are and AgI. The metal
ions have merely traded anions. All of the ions are still present after the reac-
tion; they have just “changed partners.” We can describe the reaction, pictured
in Figure 8.2, in words:
MgI
2
AgNO
3
Mg(NO
3
)
2
Magnesium iodide plus silver nitrate yields magnesium nitrate plus silver
iodide.
We use formulas to represent the substances involved in a reaction when we
write a chemical equation. In an
equation,
the formulas for reactants are placed
on the left side of the arrow and those for products are placed on the right side.
Either substance may be written first on each side of the equation:
MgI
2
AgNO
3
£ Mg(NO
3
)
2
AgI
(Not balanced)
Even better, we can write a
balanced equation,
which shows the relative
numbers of atoms of each of the elements involved. The unbalanced equation
just presented seems to indicate that an iodide ion has disappeared during the
reaction and that a nitrate ion has appeared from nowhere. As written, that equa-
tion violates the law of conservation of mass. Thus, we must always write bal-
anced equations for reactions. The word “equation” is related to the word
“equal”; an equation must have equal numbers of atoms of each element on
each side. Such an equation is said to be
balanced.
We must always write balanced
chemical equations.
Figure 8.1
Reaction of
Hydrogen and Oxygen
The bonds in the diatomic molecules
and are broken, and new
bonds are formed between hydrogen
and oxygen atoms.
O
O
H
H
H
H
H
2
O
2
H
H
H
H
O
O
